The opaque boundaries of glazed openings or apertures constitute a termination or interruption in the field of view available through the opening. This is widely recognized and accommodated in most circumstances by the ability of the observer to change position thereby varying the field of view through the opening. There are, however, circumstances where the observer's ability to change his field of view is limited and/or the field-limiting effect of the opaque boundary is of more than usually critical importance.
Window apertures often contain frames or glazing bars which act as light barriers to interrupt the field of view available through the aperture. Usually this is of little or no consequence, and the restricted field of view is acceptable; at other times it is possible, as mentioned above, to overcome the obscuration by adjusting the observation position. Furthermore, in unassisted binocular vision, if the barrier is narrower than the inter-ocular distance (about 60 mm) one or other eye can generally receive light from objects which the other eye cannot see because of the obscuration due to the barrier. In some viewing tasks, however, the ability to observe areas obscured by glazing bars or aperture boundaries does become critical. If the barrier is wider than the interocular distance and it is not desirable or possible for the observer to adjust the observation position in order to mitigate the obscuration created by the barrier the problem is exacerbated.